The only heater I really know of that can heat a tank is only suitable for 2 gallon or higher. >.< Any lower and it'll fry the fish.
He probably just darts around a lot because he's bored. As long as it's not injuring him I don't see too big of a problem. Do you have anywhere for him to hide in the tank?
Edit: And every 3-4 days might not be enough for a 1 gallon. The ammonia builds up in the tank and can burn the fish, including the sensitive gills. If your tank is constantly below 76 degrees and you've had the fish for a month, then he's probably just slowing down because he's cold and his metabolism is slow. What kind of food are you feeding him?